100% Bluff Body Drag Reduction Theory

[aerohead]

Originally Posted by aerohead

The car guys always defer to 'drag-factor',or,CdA.And Cds are always in the context of frontal projected area.
Since most modders aren't going to chop the roof off their car or do extreme alterations to frontal area,we've kinda defaulted to drag coefficient as the thing of primary interest.
Jaray got the whole 'Streamlined body of revolution in ground reflection' going back in 1922.
Jaray,Kamm,Lay,and so many others ended up using the body of revolution as their starting point,it seems like the convention with road vehicles.
Hucho used it at Volkswagen and he's the one who cited the L/D 2.5:1 Streamline form,or elliptical form as the 'minimum' for drag.
He chastised Mercedes_Benz for not honoring it with R&D of their C-111 III.
I've been looking for anything which demonstrated the lowest Cd with minimum structure and so far,for something in ground-effect,which cannot benefit from the jet pumping action of a body in free-flight,I've never come across anything "shorter" than L/D 2.5,which would translate to a vehicle with Length,5X it's height.
Al's closing gotta go,will pickup tomorrow.

With respect to streamlining,the authors of fluid dynamic texts,when referring to automobiles say a 'hemispherical' nose(half of it) is adequate below 250-mph.They say the rear part of the vehicle is most important.Moving the point of separation back is the key,and the only way you can do that is with a tapering tail.If a 2.5:1 teardrop tail section is the "shortest" separation-free "tail",then perhaps it's a safe model for us.It worked for Jaray.It worked for Kamm.It worked for Lay.It worked for Reid.It worked for Heald.It worked for R.G.S.White.It worked for Hucho.It worked for AeroVironment.
I'd like to see the 3.33 form.So far,nobody has posted a photo of it.And I would like to know the context in which it's performance figures were derived.
When a prosecutor makes a claim against a defendent,they provide a brief of particulars.It's only common courtesy.

[aerohead]

Originally Posted by Christ

I was just pondering this point earlier today and part of last night... I wonder though, if the ideal really should be 5:1 in ground effect, or if ground effect changes some part of the design?

In other words, if a body of revolution of L/D 2.5:1 were placed in ground effect without being halved, would the 2.5:1 fineness ratio still be most effective, or is that the most effective ratio for an airship-type vehicle?

The research by the PhDs says that a vehicle with L/H= 5,@ zero-ground clearance has the lowest attainable Cd.
In "reflection".this produces a form with L/D= 2.5.This form also just happens to have Mair's 22-degree angle.Coincidence?
This form happens to look like an airship because airships are designed for low drag which is what this form delivers.
Below 250-mph,the 1/2-hemisphere nose is adequate ,with the rest of the body,a long tapering tail.(Teardrop ).
This was the premise of the Rolex-Big Ben template.
A shorter form suffers a Cd increase from profile drag.
A longer form suffers from increased skin friction.
You can clearly see this relationship in Hucho's table for ellipsoids.You can also see it in the table for sections.
There is always a point of minimum Cd where the curves for profile drag and skin friction cross.For 3-dimensional bodies this occurs @ L/D = 2.5.
In FLOW-IMAGES I attempted to provide enough examples of flow separation such that one could just look and see what happens when you go "off template."

[Christ]

Thanks again, Phil.

I'll take a look at that gallery again and see if I can glean some more information from it.

It's best to know what you're looking for before you look, lest you be confused by what you see.

[mnmarcus]

I'd like to see the 3.33 form.So far,nobody has posted a photo of it.And I would like to know the context in which it's performance figures were derived.

I believe it was a tuna.. presumably travelling in salt water at whatever speed a tuna swims at. It probably did not take into account the locamotion of the tuna being side to side tail motion (maybe skewing the results towards longer=better).

[aerohead]

Originally Posted by mnmarcus

I believe it was a tuna.. presumably travelling in salt water at whatever speed a tuna swims at. It probably did not take into account the locamotion of the tuna being side to side tail motion (maybe skewing the results towards longer=better).

It would be nice to see a 'technical' drawing of the bluefin tuna,lowest drag form of the tuna.
A plan view would give us the most important view,fixing the maximum camber point for side body flow ( where the majority of water travels ).
My texts which cover fish and shark put the maximum cross-section ( frontal area ) at approx. 1/3 rd body length,with the remainder of the fish the boat-tail portion.
The bluefin is supposed to have the lowest drag of fish.The shortfin mako is supposed to be the lowest drag shark.The sailfish is supposed to be the fastest fish.Bottlenose-dolphin,the fastest dolphin.The gentoo penguin is said to have the lowest drag coefficient ever recorded for an organic underwater structure.The perigrine falcon is the lowest drag and fastest bird,in full stoop,at around 257-mph.In India,a particular type of swift was reported at over 200-mph in level flight.
They're all teardrop or fusiform ( spindle-like ) in body structure.
One of the members shared the NUNA solar car with us.It's makers claim Cd 0.07,a remarkable acheivement! That car has every component fully boat-tailed either in plan or elevation.You can argue over it's practicality as a daily driver,crashworthiness,etc.
The 'laminar' forms,which move the max camber location aft do provide more laminar body area but it's important to remember that with cars,separated flow is the bogeyman,not skin friction.And ground-effect can compromise laminar structures.
What may seem 'fantastic' on a closed-course mileathon at 20-mph may be a non-starter in the 'real' world of driving,at 'real' velocities.

[kach22i]

Originally Posted by aerohead

One of the members shared the NUNA solar car with us.It's makers claim Cd 0.07,a remarkable acheivement! That car has every component fully boat-tailed either in plan or elevation.You can argue over it's practicality as a daily driver,crashworthiness,etc.

It does not fit the template, but the canopy does if scaled down.

http://s184.photobucket.com/user/kac...?sort=3&page=1


Maybe it's one of those outside the template wonders - lots of them.

I just found a good paper talking about some of Jaray and Klemperer's work. If you take a foil and place next to the ground the drag can increase 50% to 500%. However, if you remove the lower camber making it a flat bottom foil you can greatly reduce that drag. You can also arch the top side of the body more to similar affect!

See page 13, rest of the thesis is pretty good too.
http://epubl.ltu.se/1402-1617/2007/0...X-07007-SE.pdf

[NeilBlanchard]

Does the coefficient of drag (Cd) take surface drag into account, or just shape drag?

And the other major issue with these shapes of "cars" is they are incredibly prone to drag with crosswinds; and they often have to have all wheel steering, so it can "yaw" to face into the wind while driving on a different vector.

[freebeard]

Bug deflectors!

Well, there it is in all it's impractical glory; 100% bluff body drag reduction.
Things aren't perfect though, the plate-bluff body system reduction is only
62%

If you caught that why do you say 100% in the title? The drag of the rod is equal to an airfoil 10x the diameter, i.e. about the size of you box.

OTOH, this Willys-style hood looks like it would divert air sideways around the flat windshield.

-------

If you really want to address the forebody look to Viktor Shuaberger. This is his Biological Submarine.

[aerohead]

Originally Posted by kach22i

It does not fit the template, but the canopy does if scaled down.

Automobile 2 - Odds And Ends Photos by kach22i | Photobucket


Maybe it's one of those outside the template wonders - lots of them.

I just found a good paper talking about some of Jaray and Klemperer's work. If you take a foil and place next to the ground the drag can increase 50% to 500%. However, if you remove the lower camber making it a flat bottom foil you can greatly reduce that drag. You can also arch the top side of the body more to similar affect!

See page 13, rest of the thesis is pretty good too.
http://epubl.ltu.se/1402-1617/2007/0...X-07007-SE.pdf

Yes,many university teams have turned to airfoils as models for their racer's body.Of particular interest are the laminar profiles.
The fly in the ointment for us ecomodders is how to apply the technology to an existing passenger car.
The wing car typically requires a recumbent or supine driver position to minimize frontal area,something,perhaps most motorists couldn't live with.
And ingress/egress requires the assistance of team members.
Hard to beat on the track though!

[aerohead]

Originally Posted by NeilBlanchard

Does the coefficient of drag (Cd) take surface drag into account, or just shape drag?

And the other major issue with these shapes of "cars" is they are incredibly prone to drag with crosswinds; and they often have to have all wheel steering, so it can "yaw" to face into the wind while driving on a different vector.

Cd will embody:
*pressure drag
*surface friction drag
*internal flow drag
*interference drag
*induced drag